1. Field of the Invention
This invention relates to systems for conveying particulate material and more particularly to the field of devices for fluidizing particulate materials and conveying same while in liquid suspension.
2. Description of the Prior Art
Slurry conveying systems have long been used for disposal of waste materials in industries. From the standpoint of flow, there are three types of bins: mass-flow, funnel-flow, and expanded-flow.
Mass-flow occurs when the hopper walls are sufficiently steep and smooth to cause flow of all the solid without stagnant regions. Valleys, ledges and protrusions are not permitted in the hopper. Mass-flow bins have a first-in, first-out flow sequence. Mass-flow type bins are recommended for cohesive materials. The critical flow rate must not be exceeded to prevent channeling, that is, the development of a fast flowing column of material within the bed of material.
Funnel-flow occurs when the hopper walls are not sufficiently steep and smooth to force material to slide along the walls or when the outlet of a mass-flow bin is not fully effective. Solid flows toward the outlet through a channel that forms within stagnant material. The diameter of the channel approximates the largest dimension of the effective outlet. As the level of solid within the channel drops, layers slough off the top of the stagnant mass and fall into the channel. This spasmodic behavior is detrimental with cohesive solids since the falling material packs on impact, thereby increasing the chance of material developing a stable arch across the hopper so that a complete stoppage of flow results. A channel may empty out completely, forming what is known as a rathole.
The funnel-flow bins are more prone to cause arching of cohesive solids than mass-flow bins and may therefore require larger outlets for dependable flow. Funnel-flow bins are suitable for coarse, free-flowing or slightly cohesive nondegrading solids.
Expanded flow bins are formed by attaching a mass-flow hopper to the bottom of a funnel-flow bin. The outlet usually requires a smaller feeder than would be the case for a funnel-flow bin. This design is useful as a modification for existing funnel-flow bins to correct erratic flow caused by arching, ratholing or flushing.
Generally, the outlet dimensions of a hopper outlet are sized to assure unobstructed flow. Flow may be obstructed by interlocking of large particles and cohesive doming and ratholing across or above the outlet. Prevention of interlocking normally requires that the outlet be larger than several particle sizes. Prevention of cohensive obstructions requires that the outlet be sufficiently large to ensure the failure of potentional cohesive obstructions.
When solids flow into a mass flow conical hopper, the solids are unconsolidated when deposited at the top, but, as an element of the solid flows down, it becomes consolidated under the pressure acting on it in the bin. In a mass-flow bin, a dome across the hopper is the potential obstruction.
Many solids are free flowing if they are kept in motion but cake severely if stored at rest for a period of time. Flowability tests predict the maximum time that a solid can remain in storage at rest.
Gas can be introduced purposely into a bin to promote flow. In U.S. Pat. No. 4,085,975 to Bilkvist issued Apr. 25, 1978, particulate material is aerated as it is transferred into a pressure vessel. Ambient air coming through a bottom perforated plenum pad mixes with incoming bulk particulate material to prevent compaction or compression in the loader. During emptying of the bin, the ambient air supply to the aeration pad is closed and a blower is used to force the particulate material out of the bin.
Coal transporting systems are currently used to transport materials which include fines, such a coal dust, and particulates as large as 3" in diameter. These materials are generally conveyed to settling ponds. In certain installations, the amount of air utilized must be controlled and, therefore, only a minimum amount of air should be utilized to effect conveyance. Large amounts of water must be avoided in conveying in order to avoid creating additional problems of disposing of the water.
Systems for conveying material having a wide range of particle size which include dusty materials preferably are referred to as wet-phase, meaning that water is utilized. One of the major problems involved is to prevent arching in the area where the material must flow from the charging vessel into the conveying line because a constriction exists at that point and large particles tend to arch there with subsequent plugging of the line.
Waste materials in power generation plants are normally handled using a jet pump. Jet pumps eductors are normally used for wet conveying systems. The motive power is provided by a high pressure stream of water. The resultant jet of high velocity fluid creates a low pressure area within, sucking solid material into the device and discharging it into the conveying line.
Eductors require large volumes of water at high pressures to work properly and efficiency is low. Particle size is limited to the size of the internal ports of the eductor. Head pressure is also low, restricting the use to shorter conveying lines having little or no lift. Because of the restrictions in eductors, an 8" line is needed to pass 3" particles. This increase in line size to accommodate course materials mandates large quantities of carrier liquid, thus contributing to the inefficient operation.
Prior art blow tanks systems utilize a pressure vessel having a cone-shaped bottom. The cone-shape is designed to funnel solids to a central discharge point with minimum hangup. Arching is a common problem when conveying particles which are larger than fines or when the material to be conveyed is a mixture of fines and large particles. High velocity blow tank systems are plagued with wear problems whenever conveying abrasive materials. Wear rate is proportional to the cube of the velocity. Conventional blow tanks are unreliable for wet coal because of presistent plugging.
It is therefore an object of the invention to provide a conveying system which will convey particulate using a minimum of water and air.
It is a further object of the invention to provide a system which minimizes the arching of coarse particles at the pressure vessel discharge port.
It is a still further object of the invention to prevent the retention of solids on vessel walls after discharge and to overcome the problems of insufficient fluidizing of the particulate in pressure vessels.